This invention relates to the encoding of voice signals by differential modulation such as differential pulse-code modulation (DPCM), delta modulation, or continuously variable-slope data (CVSD) modulation. In particular, it relates to a method and means of improving the intelligibility of voice signals that are subjected to differential modulation.
CVSD modulation is a well-known system for producing digitally encoded signals from analog signals. To perform the encoding, the analog signal is sampled at spaced intervals. In each of these intervals a reconstructed signal is compared with the analog signal. If the analog signal is bigger, a logical one is generated; if the analog signal is smaller, a logical zero is generated. The resulting bit stream is integrated in time to determine the signal that is compared to the analog signal. As described so far, the system is Delta Modulation which features integration with a constant slope. When that slope is made variable according to a rule, the result is CVSD modulation. The rule is normally applied to cause a change in the slope of the comparison curve after a predetermined consecutive number of intervals indicate a change in the same direction.
The principal objective of using CVSD modulation is to encode voice signals for broadcast at relatively low bit rates. In such a system, the bit rate is normally fixed, and it is necessary to design the operating characteristics of the system to deal effectively with a desired dynamic range and a desired frequency spectrum. It is evident that for a fixed signal level the maximum slope of an audio signal is a function of the high-frequency components in the signal and for a given audio waveform the maximum slope is a function of the strength of the signal. This is summarized in the statement that CVSD modulation has a decrease in the usable dynamic range with an increase in frequency, and vice versa. In contrast, there is no particular correlation between the loudness or softness with which a speaker addresses a microphone and the frequency content of the signal that he or she delivers to the microphone.
It is an object of the present invention to match the characteristics of different speakers more closely to the characteristics of CVSD modulation.
It is a further object of the present invention to improve the intelligibility of voice signals that are subjected to CVSD modulation.
Other objects will become apparent in the course of a detailed description of the invention.